In the continuing search for organic polymers suitable for use at elevated temperatures, many different repeating structures involving diverse connecting linkages between aromatic moieties have been suggested, e.g., aromatic structures connected by linkages such as imides, ethers, sulfones, ketones, etc. Unfortunately, as potential performance at elevated temperature has been enhanced, the amenability of the polymer candidates to classical molten techniques of polymer fabrication has declined or disappeared. More often than not, the same decline in melt processability accompanies attempts to produce high temperature stable polymers having an elongation of at least about 50%, a necessary property for many polymer applications, e.g., if the polymer-insulated wire is to be capable of being twisted about itself without cracking of the insulation.
Aromatic polyketones are known to enjoy relatively good resistance to thermal degradation. Bonner, in U.S. Pat. No. 3,065,205, suggested the Friedel-Crafts catalyzed polymerization of certain reactants to yield polyarylketones, and lists as typical Friedel-Crafts catalysts ferric chloride and boron trifluoride. The two basic reactions taught by this patent can be summarized as follows:
(1) n(HR--O--RH)+n(Cl--A--Cl).fwdarw.nHCl+H(R--O--R--A).sub.n Cl PA1 (2) n(HBH)+n(Cl--A--Cl).fwdarw.nHCl+Cl(A--B).sub.n H
and
where HBH is a polynuclear aromatic hydrocarbon such as naphthalene, HR--O--RH is a diaromatic ether such as diphenyl ether, and Cl--A--Cl is a diacyl chloride such as terephthaloyl chloride or phosgene. When phosgene and diphenyl ether are reacted, the resulting polymer will comprise repeating units of the structure: ##STR7##
The same repeating unit is taught in British Pat. No. 971,227 to arise from the self-condensation of diphenyl ether-4-carbonyl chloride, and from the reaction of diphenyl ether with diphenyl ether 4,-4'-dicarbonyl chloride.
A different approach is taken by Farnham and Johnson in British Pat. No. 1,078,234. Here, polyarylethers are produced by reaction of an alkali metal double salt of a dihydric phenol with a dihalo benzenoid compound. The dihydric phenol may contain a keto group--thus, 4,4'--dihydroxy benzophenone is claimed to give rise to a polyarylether polyarylketone (See for example, claim 15 of the British Patent).
A number of patents dealing with the improved methods of making polyarylketones have issued. Thus, for example, processes disclosed in U.S. Pat. Nos. 3,441,538 and 3,442,857 derive advantage by the use of hydrogen fluoride-enhanced boron trifluoride catalysis, a catalyst system taught in Boron Fluoride and its Compounds as Catalysts, etc. Topchiev et al, Pergamon Press (1959), p. 122; J. Org. Chem 26 2401 (1961); and I & E Chem. 43, 746 (1951). A further patent dealing with an improved process for synthesising polyarylketones is British Pat. No. 1,086,021.
British Pat. 1,086,021 discloses the Friedel Crafts type condensation polymerization of diacid halides with a second compound containing at least two displaceable aromatically bound hydrogen atoms, preferably the diacid halide and the second compound being present in substantially equimolar amounts; or of a single compound containing both an acid halide group and at least one displaceable aromatically bound hydrogen atom. It is further disclosed that molecular weight control can be achieved by using non-stoichiometric amounts of the two compounds or by adding a third component which is monofunctional under the conditions of the reaction. Monoacid halides are mentioned as examples of suitable monofunctional molecular weight control agents.
British Pat. No. 1,109,842 discloses the polymerization reaction of aryl disulphonyl halides with compounds containing at least two displaceable aromatically bound hydrogens. In all the examples of this patent, as in those of U.S. Pat. No. 1,086,021, either equimolar amounts of electrophile and nucleophile or an excess of the electrophile (i.e., diacid halide) is used. This patent further teaches the quenching of residual sulphonyl chloride groups by post polymerization addition of bases such as aniline, sodium carbonate or diphenylether as "residual sulphonyl chloride groups on the polymer chains may cause the products to suffer from rising viscosity when molten".
The foregoing teaching is incorporated herein by reference to illuminate the background of this invention. In my previously filed, copending, commonly assigned application U.S. Ser. No. 451,121, which is a Continuation-In-Part of application Ser. No. 218,465, I disclose the preparation of polyarylketones of mean inherent viscosity (0.1% W/V in sulfuric acid) 0.8 to about 1.65. The inherent viscosity and likewise molecular weight is controlled by the judicious use of selected nucleophilic agents whose reactivity to acetylation (relative to a benzene reactivity equal to 1) is greater than about 150.
The use of nucleophiles as molecular weight control agents in polymerizations of this type is preferred since, in addition to the undesirable hereinbefore mentioned melt stability problem (which results from the presence of excess electrophile which in turn results in residual acid halide polymer chain end groups) it has been found that the presence of an excess of acid halide groups (i.e., electrophile) during polymerization leads to the formation of branched chain polymers due to ortho-acylation of nucleophilic interior segments (such as diphenyl ether moieties) in a polymer chain by the terminal acid halide groups of neighboring chains, i.e., while numerous para sites on the nucleophile are still available linear chain building will occur but once such sites become low in concentration the ordinarily much slower acylation reactions at the ortho position becomes significant.
This problem has been discussed in Angelo, et al (U.S. Pat. No. 3,767,620) which describes the formation of 9-phenylenexanth-hydrol residues ##STR8## through ortho acylation during the preparation of a polymer having the repeating structural formula: ##STR9## Said residue is formed by the reaction of a diacyl halide with diphenyl ether such that at least some of the diacyl halide acylates diphenyl ether residues in each ring ortho to the oxygen atom (i.e., virtually all the para positions are already blocked by prior acylation). Such ortho acylated moieties are indicated as leading to thermal instability, specifically, impaired melt processability. Such degradation can be reduced by hydrogenation (reduction) of the polymer, for example, with ethanol and hydrochloric acid, formic acid or preferably triethyl silane in homogeneous acid media, to give the much more stable 9-phenylenexanthene residue. This reduction purportedly leads to products of lighter color and much improved melt stability due to the removal of the hydroxyl group and its replacement by a hydrogen atom.
Treatment of the above branched polymer, dissolved in dichloroacetic acid, with triethyl silane is also recommended by Agolino in U.S. Pat. No. 3,668,057 as a means of stabilizing branched chain residues.
Of course, if as I have taught in my application Ser. No. 451,521, filed Mar. 15, 1974, the polymerization is carried out with the nucleophilic agent in excess, and/or molecular weight control is effected with a nucleophilic compound, then an excess of para sites on the nucleophilic compound is available and the above-mentioned deleterious branching reaction will not occur.
However, I have now discovered that when the terminal group on either or both ends of each polymer chain is a phenoxy (or other nucleophilic) moiety having a para position available for reaction then, in highly acid media such as hydrogen fluoride boron fluoride mixtures, a preferred polymerization reaction media, another heretofore unknown branching reaction occurs. It is believed that such branching results from the reaction of the phenoxy (or other nucleophilic) groups (presumably activated, i.e., protonated by the acid media) with carbonyl groups in the polymer itself leading, I believe, to the formation of trisaryl carbonium salts. In addition to the deleterious effect of branching per se on processability, such salts are thermally very unstable and lead to degradation and discoloration in the polymer when molten.
As disclosed in my copending, commonly assigned application Ser. No. 603,635, filed Aug. 11, 1975, treatment of the polymerized reaction media with certain bases before isolation of solid polymer substantially improves the thermal stability of polymers prepared by Friedel-Crafts condensation polymerization through a controlled decomposition of ketonic and other complexes formed with the catalyst. However, even this post polymerization treatment does not, of course, preclude the formation of branched polymer chains by ortho acylation which can lead to poor reproduceability in the control of the molecular weight of the polymer and hence a certain degree of unpredictability in extrusion performance.
Moreover, this treatment does not address itself to the hereinabove mentioned strong acid catalysed protonated phenoxy group branching reaction which takes place concurrently with the polymerization reaction itself and can continue as long as the polymer is in a strongly acid media.
Thus a need exists for a method of preparing polyarylketones and polyaryl sulfones by a Friedel-Crafts condensation polymerization reaction which yields linear, unbranched products of reproducible molecular weight, stable and predictable melt viscosity, and enhanced stability in highly acid solutions. The term acylation as used in the instant application connotes the reaction in acidic media of the moiety ArCO.sup.+ or ArSO.sub.2.sup.+ with an aromatic nucleophile wherein Ar represents the residue of a protonated monomer or a polymer chain which is continuing to undergo polymerization (chain growth) by means of the acylation reaction. The term acid halide therefore connotes any reactive species forming the moiety ArCO.sup.+ or ArSO.sub.2.sup.+ under Friedel-Crafts reaction conditions. Common examples include AR--COCl, Ar--COOH and Ar--COOR and the sulfonyl analogues thereof.